I. Field of the Invention
The present invention relates generally to a method and apparatus for grinding and, more particularly, to such a method and apparatus of the type wherein a rotating grinding wheel is advanced toward a workpiece mounted on a spindle.
II. Description of the Prior Art
The previously known grinding machines typically comprise a circular grinding wheel adapted to rotate about an axle and a spindle adapted to hold a workpiece at a position radially spaced from the grinding wheel. The spindle also rotates the workpiece about its axis.
The grinding wheel is mounted on a table which is radially movable so that the grinding wheel can be moved into contact with the workpiece on the spindle. Contact of the grinding wheel with the workpiece, of course, initiates the grinding operation and removes material from the workpiece in the conventional fashion. The rotation of the workpiece on the spindle is necessary to insure that the material removed from the workpiece is evenly distributed around the workpiece, thus, grinding a true diameter of the workpiece.
Although any of a number of means may be utilized to advance the table supporting the grinding wheel toward the workpiece on the spindle, typically a threaded shaft cooperates with a threaded boss on the table. Consequently, rotation of the threaded shaft in a first rotational direction advances the grinding wheel toward the workpiece while, conversely, rotation of the shaft in the other direction retracts the grinding wheel from the workpiece. Likewise, the speed of rotation of the shaft is directly proportional to the lineal or radial speed of the grinding wheel toward or away from the workpiece.
In the previously known grinding machines, the grinding wheel is advanced towards the workpiece either manually or at a constant and predetermined lineal rate obtained from a steady state rotation of the shaft.
Conventional gauging means contact the outer periphery of the workpiece and provide an electrical output signal representative of the diameter of the workpiece. When the workpiece has been ground down to a predetermined diameter, the advancing means for moving the grinding wheel towards the workpiece are stopped.
Many previously known grinding operations typically comprise two separate grinding operations, namely a rough grinding operation and a finish grinding operation. In the rough grinding operation, the grinding wheel is advanced towards the workpiece at a given lineal speed, for example, 0.004 inches per second which removes most of the excess material from the workpiece. The finish grinding operation occurs subsequently and in this operation the grinding is also moved towards the workpiece in the above-described fashion but at a much slower rate, typically 0.0005 inches per second. A lesser amount of material is removed from the workpiece during the finish grinding operation than in the rough grinding operation but a smoother finish on the workpiece and more accurate control of the workpiece diameter is obtainable during the finish grinding operation.
With these previously known grinding methods and machines, as the grinding wheel contacts and is moved into the workpiece on the spindle, the workpiece bends or deflects away from the grinding wheel and vice versa from the force exerted on the workpiece by the grinding wheel. The amount of total deflection of the workpiece, the grinding wheel, and the associated components, of course, depends upon the grinding machine and the material and construction of the workpiece. However, for a predetermined grinding machine and type of workpiece, the amount of total deflection for a given grinding feed rate remains fairly constant.
During a grinding operation, as the grinding wheel moves initially towards and into the workpiece, a relatively small quantity of material is initially removed from the workpiece due to the aforementioned total deflection despite a constant advance of the grinding wheel into the workpiece. Only when the workpiece, the grinding wheel, and the associated components are fully deflected will the grinding wheel remove material from the workpiece at the rate of advance of the grinding wheel.
This previously known grinding method is disadvantageous in that only a relatively small amount of material is initially removed from the workpiece as the grinding wheel contacts the workpiece. Since the rate of advance of the grinding wheel towards the workpiece is relatively slow, this phenomena unnecessarily prolongs the time required for a single grinding operation. For example, a typical grinding operation requires approximately eleven seconds.
A still further disadvantage of these previously known grinding methods is that due to the total deflection, even when the advance towards the workpiece is halted, the workpiece and grinding components continue to straighten out, i.e., to move toward an undeflected condition. This, of course, results in the continued removal of material from the workpiece and often results in removing more material from the workpiece than tolerances permit.
A still further disadvantage of these previously known grinding machines and methods is that due to the prolonged grinding time required by the previously known machines, the workpiece becomes heated by the friction and expands accordingly; consequently, when the workpiece cools after the grinding operation, the shrinkage incurred during the cooling often renders the workpiece smaller than the tolerance levels permit so that the workpiece must be scrapped.
The previously known grinding machines and methods, thus, suffer two primary disadvantages. First, such machines and methods are time consuming in operation and second, these prior machines and methods form a large percentage of improperly formed workpieces. Improperly formed workpieces are scrapped.